The present invention relates generally to electric vehicles, and more particularly, to a system and method for controlling DC bus voltage ripple when using a six-step (or other step) mode of operation to control inverters used in electric vehicles and hybrid electric vehicles.
The assignee of the present invention designs and develops electric vehicle charging propulsion systems. In electric vehicle and hybrid electric vehicle applications, different subsystems can be connected to the same high-voltage bus. If a subsystem draws or sinks a large current ripple component, the DC bus voltage ripple can become substantial. This, in turn, can consequently cause faulty operation of other DC bus supplied subsystems.
The main cause of high ripple current on the DC bus is the main propulsion inverter. This is especially true using a six-step (or other step) mode of inverter operation, a known method that changes the output of the inverter in discrete steps, causing a large, low-frequency current ripple component to be drawn from the DC bus. This mode of inverter operation is necessary to provide maximal output motor torque. The amount of current ripple drawn from the DC bus can be reduced if large, bulky, low-frequency input filters are placed across the input of the main propulsion inverter. However, this results in a bulky, heavy, and costly power electronics subsystem, which occupies a significant portion of vehicle space.
Another approach is to completely disable the inverter six-step mode of operation when there is substantial DC bus voltage ripple. If the six-step mode is permanently disabled, the maximal achievable torque is significantly reduced, especially at high speeds, and the vehicle acceleration time and overall performance deteriorate significantly. This is not a desirable alternative.
It would therefore be desirable to have a control system and method for controlling DC bus voltage ripple when using a step mode of operation to control inverters used in electric vehicles and hybrid electric vehicles.
The present invention comprises a system and method for controlling DC bus voltage ripple of an inverter used in an electric or hybrid electric vehicle when using a six-step (or other step) mode of operation to control the inverter.
An exemplary system comprises a torque command input for receiving an applied torque command and a sensor for measuring an average DC bus voltage. A processor compares the average DC bus voltage to low voltage and high voltage thresholds defining predetermined conditions, and conditionally enables the step mode (preferably the six-step mode) based upon the applied torque command if the voltage conditions are satisfied. If, at any time while the voltage conditions are satisfied the torque command is above a high torque threshold, the step mode is indefinitely enabled until the voltage conditions disable the step mode. If the torque command is not above the high torque threshold, the step mode is enabled for a limited time duration, and then disabled after expiration of the limited time duration or after the voltage conditions disable the step mode, whichever occurs first. If disabled due to the voltage conditions not being satisfied, then the step mode will be enabled when the voltage conditions are again satisfied.
An exemplary method comprises the following steps. An average DC bus voltage is measured. The average DC bus voltage is compared to low voltage and high voltage thresholds defining predetermined conditions. If the voltage conditions are satisfied, the step mode is enabled based upon an applied torque command. If the torque command is higher than a high torque threshold, the step mode is indefinitely enabled as long as the voltage conditions remain satisfied. Otherwise, the step mode is enabled for a limited time duration, and then disabled.
As a result of using the present system and method, maximal torque can be produced by the electric motor driven by the main propulsion inverter allowing the six-step mode of operation, without inducing faulty operation of the other power electronics subsystems connected to the same DC bus. Small high-frequency input filter can be used at the inverter input, thus reducing the size and cost of the inverter. Furthermore, by not permanently disabling the six-step mode, the maximal achievable torque may be provided by the inverter, especially at high speeds, and vehicle acceleration time and overall performance is not limited.